(1) Field of the Invention
The present invention relates to the method and device for stabilizing operation point and optical output of external optical modulator, which is used for an optical communication system. More particularly, the invention relates to the method and device for stabilizing the operation point and optical output of external optical modulator, which provides a means of regulating direct current bias automatically that determines the operation point of the modulation curve of external optical modulator.
(2) Related art statement
Corresponding to the increasing demand for recent high-speed and high-capacity information communication, optical communication has been attracting attention. In optical communication system, the method is taken, which converts the information of electrical signal array, which is to be transmitted, into a light signal array, sends it over a long distance mainly by optical fiber and then restores the transmitted light signal array to electrical signal array in long distant place.
Optical modulator is used as the means of converting the electrical signal array into the light signal array. In detail, various optical modulators are utilized depending on the speed of signal array used, the transmission distance and the light wavelength. For example, direct modulations of laser diode (LD), electric field absorption form (EA) modulators or external optical modulators, which modulate the light from CW (Continuous Wave) laser by using the optical modulator (LN optical modulator) with optical guide like Mach-Zehnder (MZ) type on the surface of the base plate that is made of materials with electro-optic effect, such as lithium niobate (LN), have been used.
Among other things, LN optical modulator is known widely as light control element which is suitable for the optical communication in broadband frequency as it can work at extremely high speeds, it doesn't have any light wavelength dependency and furthermore, it is easy to be controlled the chirp amount. Particularly it is used as the optical modulator that is suitable to the dense wavelength division multiplexing (DWDM) and high-speed communication.
However, for the LN optical modulator, there is a phenomenon (DC drift phenomenon) that the modulation curve of optical modulator moves along an applied voltage axis, depending on the variation in temperature of environment, or on the applied direct current (DC) voltage that sets the operation point (it is the voltage that is the standard for modulation signal, and usually, the signal voltage for modulation, the center of which is the voltage set as an operation point, is applied) of LN optical modulator's modulation curve (it is the curve that shows the optical output variation from optical modulator in response to the applied voltage to optical modulator).
Therefore, a method for stabilizing the operation point of optical modulator and constantly controlling the amount of output-light has been used.
One of the methods for stabilizing the operation point of optical modulator, for example, is to superimpose a low-frequency signal onto signal array and impress it to the optical modulator, extract the said superimposed low-frequency component from the optical output of the optical modulator, detect the gap of operation point by comparing the phase difference between the low-frequency signal before superimposed and the low-frequency component extracted from the said optical output and give feedback control to the value of DC voltage applied to the optical modulator so that the amount of the said gap may be reduced to zero.
Another example of the method is to monitor the amount of output-light from optical modulator and then control the applied DC voltage so that the amount of the said output-light may become constant. Moreover, based on the above-mentioned methods, various types of supplemental method and technique have been proposed, such as how to detect the amount of the operation point gap with a high degree of accuracy.
FIG. 9 is one of the examples of the conventional practice, in which the low-frequency of frequency f is superimposed in opposite phase onto applied voltage V at top A and bottom B of the modulation curve of optical modulator. Optical output P at the ideal operation point (point A, B) has only low-frequency component of frequency 2f, as indicated in the figure, and the superimposed frequency f component has disappeared. Thus, in order to minimize the frequency f component that is contained in optical output, the DC component applied to optical modulator is controlled.
In the above-mentioned method, since a low-frequency is added in at top A and bottom B where a modulation is saturated, it is necessary to increase the amplitude of the applied low-frequency voltage in order to detect the optical output variation from optical modulator easily. Therefore, the problem of deformation of wave pattern of the electrical signal array, which are the original data, has been brought about.
Also, as indicated in FIG. 10, the radiation light from LN type optical modulator, as an external optical modulator, is detected by using the photodiode (PD), which is the optical detector that is located adjacent to the LN type optical modulator, and the detection signal corresponding to the optical output from the optical modulator is outputted. Alternatively, it is possible to diverge some of the output-light from the LN optical modulator, detect it directly and obtain the detection signal corresponding to the optical output from the optical modulator.
The detection signal outputted by the photodiode is compared to standard value Vref, which is set up separately, and then the DC voltage applied to the LN type optical modulator is controlled so that the both may correspond to each other. In the conventional practice as mentioned above, in case there is the optical output variation of the light source itself or the transmission factor variation in LN optical modulator, there occurs a gap between the standard value and the optical detection output at the ideal operation point in the modulation curve of optical modulator (hereinafter referred to as “ideal operation point”; although it is often the case that it means the node or the inflection point of modulation curve in particular, it herein means not only the above-mentioned, but also the center point of the variation of modulation signal, which is intended by the designer.). As a result, the adjusted operation point deviates from the ideal operation point.
The purpose of present invention is to solve the above-described problems, to control the wave pattern deformation of electrical signal array, and to provide the method and device for stabilizing the operation point and optical output of external optical modulator, which can set up the operation point of the modulation curve of optical modulator or the optical output from optical modulator stably even in case there is the optical output variation of the light source itself or the transmission factor variation in the optical modulator.